The present invention relates generally to rechargeable lithium-ion-type chemistry batteries, and more specifically to improving battery pack life of automotive Li-ion battery packs.
Lithium ion batteries are common in consumer electronics. They are one of the most popular types of battery for portable electronics, with one of the best energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. In addition to uses for consumer electronics, lithium-ion batteries are growing in popularity for automotive, defense, and aerospace applications due to their high energy and power density.
One of the advantages of use of a Li-ion chemistry is that batteries made using this technology are rechargeable. Traditional charging is done with a two-step charge algorithm: (i) constant current (CC), and (ii) constant voltage (CV). In electric vehicles (EVs), the first step could be constant power (CP).
Step 1: Apply charging current limit until the volt limit per cell is reached.
Step 2: Apply maximum volt per cell limit until the current declines below a predetermined level (often C/20 but sometimes C/5 or C/10 or other value).
The charge time is approximately 1-5 hours depending upon application. Generally cell phone type of batteries can be charged at 1C, laptop types 0.8 C. The charging typically is halted when the current goes below C/10. Some fast chargers stop before step 2 starts and claim the battery is ready at about a 70% charge. (As used herein, “C” is a rated current that discharges the battery in one hour.)
Generally for consumer electronics, lithium-ion is charged with approximate 4.2±0.05 V/cell. Heavy automotive, industrial, and military application may use lower voltages to extend battery life. Many protection circuits cut off when either >4.3 V or 90° C. is reached.
In battery-powered systems, the ability to accurately estimate the charge remaining in the battery is highly desirable, and in many cases essential. For example, in portable electronic devices such as cameras, cell phones, portable gaming systems and computers, knowing such information allows the end user to gauge how much longer they can use the device before recharging becomes necessary. In some cases, this information can prevent the end user from inadvertently losing data, a common occurrence when a camera or a computer suddenly stops functioning due to the battery becoming fully discharged. In other applications, such as electric vehicles, knowing the remaining battery capacity may make the difference between a successful trip and an unsuccessful trip, i.e., one in which the vehicle and its driver become stranded when, without providing sufficient warning, the battery becomes fully discharged. Additionally, since a battery's voltage drops as the state of charge of the battery is reduced, knowing the state of charge allows an accurate estimate to be made of the power available to the battery-operated device, e.g., an electric vehicle.
In order to accurately estimate the remaining capacity of a battery, it is critical that the full capacity of the battery be accurately known. Unfortunately, under normal use conditions such as those encountered in an electric vehicle or other battery-power device, it is difficult to accurately ascertain battery capacity. For example, in one method of determining battery capacity, the initial capacity of the battery is gradually decreased based on a variety of factors such as battery age, the number of charge/discharge cycles to date, and temperature. Unfortunately this technique does not provide a very accurate assessment of battery capacity, both because some factors are not properly taken into account (e.g., historical temperature profiles, load conditions, depth of discharge prior to each charging, charge/discharge rates, etc.) and because the effects of the errors accumulate as the battery ages. Another method of determining battery capacity is to allow the battery to become fully discharged, and then determine the capacity of the battery during charging. Although this technique can be used occasionally, using it on a routine basis can have serious repercussions since deep discharging a battery, and in particular fully discharging a battery, can dramatically shorten its lifetime. Additionally, for most battery-powered devices, especially electric vehicles, it would be extremely inconvenient to require that the user allow the battery to become fully discharged prior to charging. This would be similar to requiring that a conventional car be driven until the gas tank was dry before refilling, simply in order to determine the gas tank's capacity.
It is important to emphasize that the prior art recognizes concerns for various charging systems, particularly as to safety and degradation of a battery cell. As battery packs used for heavy use, such as automotive electric vehicles and other heavy industrial application often have thousands of battery cells, and the assembly of the cells represents a large investment of resources (money and time), issues regarding safety and degradation are even more important. In contrast, many consumer devices include a rechargeable battery cell that typically represents a relatively small fraction of total cost, and there is generally no special requirements for obtaining and replacing a battery cell, and battery cycle life is relatively less important than capacity.
For consumer applications, aging of lithium-ion battery cells is often not a factor. A lithium-ion battery in use typically lasts between 5-7 years as it loses capacity. This capacity loss manifests itself in increased internal resistance caused by mechanical stresses (e.g., volume change, side reactions and the like) and oxidation. Eventually, the cell resistance reaches a point where the pack is unable to deliver the stored energy although the battery may still have ample charge. For this reason, an aged battery can be kept longer in applications that draw low current as opposed to a function that demands heavy loads. Increasing internal resistance with cycle life and age is typical for cobalt-based lithium-ion, a system that is used for cell phones, cameras and laptops because of high energy density. The lower energy dense manganese-based lithium-ion, also known as spinel, maintains the internal resistance through its life but loses capacity due to chemical decompositions. Spinel is primarily used for power tools.
While the general statement about limited service life of lithium-ion batteries is accurate, it is understood that longevity is very much a factor of “life style” and “living conditions” experienced by the battery pack and its cells. The speed by which lithium-ion ages is governed by many factors including temperature and state-of-charge. High charge levels and elevated temperatures hasten permanent capacity loss. Improvements in chemistry have increased the storage performance of lithium-ion batteries. The voltage level to which cells are charged also plays an important role in longevity—with a value of 4.1 maximum cell voltage chosen as good trade-off between sufficient cell capacity and reduced cycle life degradation.
Generally speaking, batteries have an increased cycle life when treated in a gentle manner. High charge voltages, excessive charge rate and extreme load conditions will have a negative effect and shorten the battery life. This also applies to high current rate lithium-ion batteries. Not only is it better to charge lithium-ion battery at a slower charge rate, high discharge rates also contribute the extra wear and tear. When a battery is charged and discharged at less than 1C, cycle lifetime for high energy density cells is generally much better than higher charging/discharge levels (as measured by a discharge capacity of the battery (Ah). (Using a 0.5C charge and discharge rate would further improve this rating. Power cells are able to handle higher rate charges and discharges.) A moderate charge and discharge puts less stress on the battery, resulting in a longer cycle life. In addition, the temperature and state of charge that the battery is stored is critical to its longevity. Keeping the battery at high SOC and high temperatures should be avoided. For example, when preparing for a long period of non-use, the vehicle should be stored at a lower SOC. Although the vehicle range is reduced in a storage mode, this sacrifice in range is beneficial to extend the pack life.
The life of lithium-ion depends on other factors than charge and discharge rates. Even though incremental improvements may be achieved with careful use of the battery, the battery cell environment and the services required are not always conducive to achieve optimal battery life. The longevity of a battery is often a direct result of the environmental stresses applied.
The very considerations given above for increasing battery cycle life can often conflict with some of the requirements for long vehicle range and use of high performance battery packs used in automotive applications and other industrial scenarios. As noted above, the resource costs represented by an advanced battery pack for automotive and industrial applications are significant, and reaching close to optimal balance between long range and pack cycle life is very important. The challenges are increased because each user will have different requirements, and those requirements will vary over time. In short, there is typically not a single solution that can be established a priori for each user.
There is a need to improve cycle lifetimes for an automotive lithium-ion battery pack, particularly for adapting to a dynamic use profile for a user. A careful balance between the range required by the individual user and long term life of the battery pack needs to be created. The following invention addresses this issue and provides a novel and non-obvious solution.